Fiber amplifiers are commonly used in many applications, including telecommunications applications and high power military and industrial fiber optic applications. For example, both U.S. Pat. No. 5,946,130, issued Aug. 31, 1999 to Rice and U.S. Pat. No. 5,694,408 issued Dec. 2, 1997 to Bott et al. describe many such applications in which fiber amplifiers are employed including the processing of materials, laser weapon and laser ranging systems, and a variety of medical and other applications.
Optical fiber amplifiers are designed to increase the power output levels of the signals propagating therealong. One conventional optical fiber amplifier design is an end pumped dual clad fiber. Referring to FIGS. 1A and 1B, the dual-clad fiber 10 has a single-mode signal core 12, a multi-mode pump core 14 surrounding the signal core, and an outer cladding layer 16 surrounding the pump core for confining pump energy within the pump core such that signals propagating through the signal core are amplified. The signal core will typically be doped with one or more rare earth elements such as, for example, ytterbium, neodymium, praseodymium, erbium, holmium or thulium. In operation pump energy is coupled into the pump core at the input end 18 of the fiber. The pump energy then propagates through the pump core until it is absorbed by the dopant in the signal core, thus amplifying signals propagating through the signal core. Although dual clad fibers can have different sizes, one typical dual clad fiber includes a signal core that has a diameter of 8-10 μm and a pump core that has cross-sectional dimensions of 100-300 μm. End pumped dual clad fiber amplifiers of this size can typically reach fiber energy power levels of 115 W.
To allow the largest amount of pump energy to be coupled into the end of the fiber, the size of the pump core is generally made as large as practical. The size of the pump core, however, is limited by the requirement to maintain a significant absorption of pump energy per unit length of fiber. While a design that introduces pump energy into the end of the fiber has led to great increases in output power levels, the practical limits have essentially been reached for pump arrays of typical power output. Facing this problem, a number of alternative pumping techniques have been developed. For example, U.S. Pat. No. 5,854,865 issued Dec. 29, 1998 to Goldberg discloses a fiber amplifier having a v-shaped notch cut into the pump core through the cladding layer. Pump energy can then be reflected or refracted from one of the angled faces of the v-shaped notch so as to be injected directly into the pump core. Another technique involves the use of a fiber amplifier having portions of the cladding and the pump core removed. The fiber amplifier is then spooled between two reflective elements and pump energy introduced into the region between the reflective elements. The pump energy is then repeatedly reflected by the reflective elements in order to amplify signals propagating through the signal core. Advantageously, such techniques also permit more uniform coupling of pump energy into the fiber, in contrast to techniques that only couple pump energy into the end of the fiber.
The current techniques, while achieving some level of success, also have their drawbacks. They can require extensive and tightly controlled processing. Also, they are generally not easily amenable to volume manufacturing and scaling. Thus, it would be advantageous to provide an inexpensive optical fiber amplifier with a relatively straightforward design that is capable of being fabricated in mass quantities while addressing each of the other aforementioned features.